This invention relates to a process of producing semiconductor devices by utilizing an ion implanting technique and more particularly to a process of producing semiconductor devices by utilizing a distinct phenomenon developed in the semiconductive material of substrates due to the implanting of accelerated ions into the substrates.
Among the existing processes of producing semiconductor devices there is one process called the PLANOX (Trade Mark) or ISOPLANAR (Trade Mark) process. That process is to selectively apply a coating of silicon nitride (Si.sub.3 N.sub.4) to a surface of a semiconductor substrate and oxidize the substrate with the coating in an oxidizing atmosphere at a high temperature to selectively oxidize the surface of the substrate so that a portion of the substrate's surface having the silicon nitride coating applied thereto is not oxidized while that portion of the substrate's surface without such a coating is oxidized. The process is advantageous in that a difference in level between the surface of the substrate and the oxidized surface is small resulting in a reduction in a tendency to break an aluminum wire disposed across both surfaces later on. In the oxiding step, however, the particular oxidizing agent such as steam or oxygen can be laterally diffused into that portion of the substrate directly under the silicon nitride coating on the border of the latter to oxidize it. Thus the oxidized portion increases in volume to force up the edge of the silicon nitride coating leading to the formation of a protrusion on the oxidized surface. The protrusion is apt to break aluminum wire disposed thereacross. Also large lattice defects and high strains may occur on that portion of the substrate's surface directly contacted by the silicon nitride coating due to a difference in coefficients of thermal expansion between the silicon nitrate and the semiconductive material of the substrates. Such lattice defects and strain are undesirable from the standpoint of the characteristics of the resulting semiconductor device.
In order to reduce the lattice defects and strains as above described, it has been proposed to attempt to first apply a thin layer of silicon dioxide to a surface of a substrate formed, for example, of silicon and then dispose a silicon nitride coating upon the thin layer. In this measure oxygen or steam as an oxidizing agent involved can also be laterally diffused into that portion of the substrate directly under the edge of the nitride coating on the border of the latter to oxidize it as in the above mentioned process. Then the nitride coating and the dioxide layer are removed until the surface of the substrate is selectively exposed. During this removal of the silicon dioxide layer, that portion of the substrate initially overlaid with the nitride coating may have formed on the border a recess because the silicon has as etching rate less than silicon dioxide in dilute hydrofluoric acid commonly used in removing the silicon dioxide layer. Like the protrusion as above described, the recess is apt to break aluminum wire disposed thereacross later on.
Also the PLANOX or ISOPLANAR process can be effective for producing ultrahigh frequency semiconductor devices.
In order to improve the ultrahigh frequency operating characteristics thereof, ultrahigh frequency semiconductor devices have included a comb-shaped or mesh type emitter structure and overlay type electrodes on the oxide layer. With an increase in the operating frequency thereof, such devices tend to have a reduced width of the emitter stripe and electrode therefor while at the same time having a smaller spacing between the emitter region and the base electrodes. With a high gain required in the range of ultrahigh frequencies, an additional capacitance provided by the oxide film on the surface of the semiconductor element between each of the emitter and base electrodes and the collector electrode significantly affects the high frequency parameters because of the overlay structure thereof and may be one of the main factors of an impediment of the characteristics of the element. If the oxide film on the surface of the substrate is increased in thickness to reduce the additional capacitance then a difference in level between the film and surface becomes large which is apt to cause the breaking of aluminum wire as above described. Further the base and emitter junctions decrease in depth while the lateral diffusion along the substrate's surface decreases in length. After electrodes have been disposed on the substrate, the metal thereof reacts on the semiconductive material over a length equal to or greater than the length of lateral diffusion until the withstanding voltage is decreased. Alternatively the junctions may be broken down. In addition, the diffusion region produced by conventional processes has a curvature of diffusion with a few exceptions, for example, except for the diffusion region of mesa type semiconductor devices. An electric field established in the diffusion region is concentrated on the curved diffusion portion to limit the voltage which can be withstood by the diffusion region. Also in high frequency transistors, the distance between windows for electrodes disposed in a silicon dioxide film on the emitter and base regions greatly affects the characteristics of the transistors and is reduced as the operating frequency is displaced toward the ultrahigh frequency range. Thus a photolithographic process for forming the electrodes is required to effect the registering of masks with a high accuracy while high technical skill is necessary to adjust the etching of the silicon dioxide film. In conventional photolithographic processes, it has been possible only to describe on the silicon dioxide film a pattern with lines having a minimum width of from about 1.0 to 1.5 microns. Since ultrahigh frequency semiconductor devices have a decreased width emitter region, the ultrahigh frequency characteristics thereof are affected not only by the capacitance provided by the bottom surface of the diffusion region but also by an additional capacitance provided by the lateral surface thereof. This has imposed limitations upon the operation of the semiconductor devices in the range of ultrahigh frequencies.
In order that active and reactive elements disposed on a single silicon substrate be electrically insulated from one another, the PLANOX or ISOPLANAR process can be utilized to form silicon dioxide regions on the substrate between the elements. In a heat treatment for forming those silicon dioxide regions, the impurity distribution already formed in the substrate can be changed and the oxidation proceeds not only in the direction of the depth of the substrate but also in the lateral direction thereof. This gives rise to serious problems in designing and manufacturing integrated circuits and impedes the high density assembly of active and reactive elements.